how to find valency in an element

rite answer 

For neutral atoms, the number of valence electrons is equal to the atom's main group number. The main group number for an element can be found from its column on the periodic table. For example, carbon is in group 4 and has 4 valence electrons. Oxygen is in group 6 and has 6 valence electrons.

Valency can be defined as the number of electrons in the valence shell(last shell or ring).
For any element to attain valency, it has to have maximum number of electrons in its valence shell. This can be achieved either by donating its extra electron to another element of by receiving few electrons from another element(s).
Number of electrons in the valence shell to attain stability can be expressed by using the formula 2k^​2.
                                                                                                                                                             where 'k' stands for the place of the shell.
For eg. the number of electrons to attain stability in the first shell will be,
2k​²
=2*(1)²
=2*1
=2
By using this formula, we can conclude that the no. of electrons in the valence shell to achieve stability must be, 2,8,18, etc.
So if they give u an element, say chlorine, you have to first think of its atomic number i.e., 17 and its atomic configuration- 2,8,7.
For the third shell, we should actually take 8 as its stability point. Therefore, we can conclude by saying that, chlorine requires 'more' electron to gain stability and that 1 electron is its valency.
 

If the outer shell has 7 electrons then the valency is 1
''   ''     ''        ''      ''   6  electrons  ''      ''   valency is 2
''   ''     ''        ''      ''   5 electrons   ''      ''   valency is 3

''   ''     ''        ''      ''   3 electrons   ''      ''   valency is 3
''   ''     ''        ''      ''   2 electrons   ''      ''   valency is 2
''   ''     ''        ''      ''   1 electrons   ''      ''   valency is 1